The invention relates to a method for working up an aqueous, alkaline-alcoholic and lignin containing solution. The invention relates in particular to a method for the preparation of lignin from aqueous solutions, which are obtained in the cleavage of lignocellulose.
In connection with the shortage of crude oil, the renewable resource lignocellulose (straw, wood, paper waste, etc.) has been gaining more and more importance as a starting material for chemical products and fuels. Lignocellulose consists of the ultra-structurally cross-linked polymeric main components cellulose, hemicellulose and lignin, frequently accounting for about 85-90% of the raw material. The remaining amount may be summarized under the term low-molecular ingredients. Among these ingredients, there is to be stressed, apart from extract materials and inorganic by-products, especially the acetate. Acetyl groups are present especially in deciduous trees and grasses as well as other lignocelluloses. In the majority of cases, these are chemically bound to hemicellulose, more precisely, in the polymer O-acetyl-4-O-methyl glucurono(arabino)xylane. The acetate is of significant importance in numerous ways. As a recyclable material, it may be prepared in the form of acetic acid by way of distillation from cellulose pulping liquor of deciduous tree celluloses. In the preparation of bio-alcohol from lignocellulosic substrates such as, e.g. straw, acetate is a strong inhibitor, negatively affecting the yield of alcohol.
The cleavage of the components present as polymers and the separation thereof into individual product flows as well as the further processing thereof into high-order products is the task of biorefineries of the biochemical platform. The rentability of such biorefineries is largely dependent on the value creation on the basis of the product flows.
Apart from the use of the carbohydrate portion, also the amount and the quality of the obtained lignin exert a strong influence on the value creation of the entire process. Lignin is steadily gaining economic importance as a substitute for petrochemically produced aromatics. The possibilities of use of the prepared lignin, however, are significantly dependent on its chemical composition, and especially on the molecular weight of the prepared lignin fraction. In particular sulfur-free low-molecular lignin fractions are very in demand as a raw material for utilization in the production of plastics and resin. In contrast to Kraft or sulphite cellulose plants, biorefineries aim at the preparation of sulfur-free lignin.
Among the methods used in biorefineries for working up lignocellulose, there are to be mentioned especially alkaline methods, the work-up principle of which being primarily the removal of the lignin. Especially deciduous wood, straw, bagasse or annual and perennial grasses have proven to be rather advantageous as lignocellulose substrates. The chemical principle this is based upon is the alkaline deesterification, by means of which the bonds between lignin and hemicellulose as well as also acetic acid-hemicullose esters are cleaved. The additional use of an alcohol, with ethanol being preferably used, improves the solubility of lignin, which enables lower reaction temperatures. Such a method was described by Avgerinos and Wang, and in 1981 the patent was filed (U.S. Pat. No. 4,395,543). Therein, there is claimed a method for degrading lignocellulose, wherein there is used an extraction solution consisting of water, having 40 to 75% alcohol and a pH of 11 to 14. Additionally, in the patent it is indicated that the quantity of the released hemicellulose cleavage products decreases with increasing content of alcohol. Thus, degradation solutions having a high alcohol content achieve more selective results.
Extraction solutions from lignocellulose degradation with alcohol, water and base thus frequently show high pH values in the range from 12 to 14, comprise alcohol concentrations in the range from 40 to 75% and temperatures from 40 to 90° C.
An essential economic factor of the alkaline-alcoholic degradation method is the way how the alcohol is being recovered. Primarily, this is realized by way of thermal recovery from the degradation solution. An essential saving potential is the reduction of the quantity of the solution to be thermally treated, via separating off the alcoholic degradation solution as selective as possible.
While via ultrafiltration of lignin solutions, e.g., by using ceramic membranes, lignin fractions are separated with a cut-off of 5, 10 or 5 kD, respectively, (A. Toledano et al. Chemical Engineering Journal 147 (2010) 93-99), nanofiltration further offers the possibility to separate fractions having a low molecular weight. With regard to hemicellulose components in alkaline process waste waters in cellulose industry, R. Schlesinger et al. (Desalination 192 (2006) 303-314) reports a nearly complete separation of molecules having more than 1 kD.
In nanofiltration, there are used different membrane materials having different membrane structures. Inorganic ceramic membranes are composed of homogenous material and show a change in pore structure via the membrane thickness. Rather frequently composite membranes are used, wherein the active membrane separation layer is decisive for the separation effect. As membrane materials for this separation layer, there are frequently used polyether sulfone membranes, polypiperazine membranes as well as aromatic polyamide membranes.
In the field of paper and cellulose, nanofiltration was, for example, used in WO 02/053783 and WO 02/053781 in order to enrich xylose in the permeate of the nanofiltration from the “black liquor” of a sulphite process. In WO/2007/048879 there is described a nanofiltration process for separating xylose from a biomass hydrolysate having a high dry matter content.
Further there is reported the preparation of monosaccharides from alkaline black liquor by way of nanofiltration (US 2009/0014386 A1).
In US 20060016751 there is described a nanofiltration method for concentrating extract and waste material streams of a paper factory.
In the last couple of years, the interest in the preparation of lignosulfonates from the pulp of Kraft cellulose processes has increased considerably. A.-S. Jönsson et al. (2008: Chemical Engineering Research and Design, 86, 1271-1280) used a combined process of ultrafiltration/nanofiltration and concentrated the lignin content of the aqueous alkaline solution from 60 g/L to 165 g/L. They used a series of membranes, consisting of polyether sulfone, polysulfone and polyvinyl fluoride, at 60° C.
Nanofiltration with polyether sulfone and polysulfone membranes was further used for recovering sodium hydroxide from strongly alkaline solutions (about 200 g/l NaOH) in a cellulose fibre production. The tests were carried out in the temperature range from 20 to 50° C. in a laboratory agitator cell. The obtained retentates for hemicellulose were in the range from 50 to 95% (Schlesinger 2006). In the course of this, a polyether sulfone membrane was tested, which proved itself well under aqueous-alkaline conditions. That membrane was also tested under strongly alkaline-alcoholic conditions of the method according to the invention, and showed to be unstable in the temperature range above 40° C.
For aqueous-organic solutions having a high solvent part, modified polyimide membranes were used. These membranes, however, showed to be pH-unstable in the alkaline range.